1. Field of the Invention
The present invention relates to a motor equipped with a bearing member mounted on a bearing holder, wherein the bearing member is moveable in an axial direction and immovable in a radial direction.
2. Related Background Art
A motor may be provided with a rotor shaft that is rotatable with a rotor unit in an unitary fashion, wherein end sections of the rotor shaft are rotatably supported by appropriate bearing members. For example, FIG. 5 shows a permanent magnet type (PM type) stepping motor of the type described above. The stepping motor includes a stator unit with a core assembly 3 having a plurality of coils 2 that are wound in a generally cylindrical shape about a rotor shaft 1. The coils 2 are disposed adjacent to each other along an axial direction to form a plurality of phases. A rotor section 4 is mounted on the rotor shaft 1 in a center side section of the core assembly 3 in a manner that the rotor section 4 is located in close proximity to yokes 5 of the core assembly 3 in the radial direction. The rotor shaft 1 has a base end section (a right end section in the figure) that is rotatably supported by a pivot bearing member 6. Further, a lead screw 7 is formed on an output side (a left side in the figure) of the rotor shaft 1 in a unitary fashion. An end section (a left end section in the figure) of the lead screw 7 is also rotatably supported by a bearing member (omitted from the drawings).
The pivot bearing member 6 is equipped with a main body section 6a in a cylindrical shape. The main body section 6a of the pivot bearing member 6 is supported by a bearing holder 8 in a manner that the main body section 6a is moveable in the axial direction but held immovable in the radial direction. The bearing holder 8 has a mounting hole 8a, which is a through hole extending through the bearing holder 8, provided generally in a central section of the bearing holder 8, as shown in FIG. 6. The main body section 6a of the pivot bearing member 6 is inserted in the mounting hole 8a of the bearing holder 8. The main body section 6a of the pivot bearing member 6 is mounted on the bearing holder 8 in a manner moveable in the axial direction in order to assemble the pivot bearing member 6 in close contact with the rotary shaft 6, such that the rotary shaft 1 is highly accurately supported without looseness. The structure that moveably supports the main body section 6a in the axial direction is described in greater detail below.
The bearing holder 8 and the pivot bearing member 6 are retained in place by a flexible supporting member 9 that is made of a plate-shaped flexible spring member as shown in FIG. 7, for example. The flexible supporting member 9 is attached to a housing member of the stator section in a manner to cover the bearing holder 8 from a rear end side (a right end side in FIG. 5) of the housing member of the stator section. In particular, when the bearing holder 8 is formed from a resin material, the bearing holder 8 with the pivot bearing member 6 may be held by a fixing structure in which the bearing holder 8 with the pivot bearing member 6 is sandwiched between the yoke 5 of the stator section and the flexible supporting member 9.
More specifically, the flexible supporting member 9 is provided with four mounting sections 9a in the form of hooks provided at four side sections of the flexible supporting member 9, respectively. The mounting sections 9a pass side sections of the bearing holder 8 and engage sections of the yoke 5 of the stator section, respectively. As a result, the bearing holder 8 made of a resin member is held and affixed between the flexible supporting member 9 and the yoke 5.
Furthermore, a section of the flexible supporting member 9 is cut out in the flexible supporting member 9 and bent to form a force-application spring section 9b. The force-application spring section 9b is provided generally in a central section of the flexible supporting member 9 at a location where the force-application spring section 9b abuts against a rear end face of the main body section 6a of the pivot bearing member 6. The force-application spring section 9b applies a pressing force to the pivot bearing member 6 in the axial direction. By the axial pressing force caused by the force-application spring section 9b, the entire pivot bearing member 6 is pressed toward the rear end surface of the rotary shaft 1. As a result, the pivot bearing member 6 is positioned in the axial direction.
However, when the bearing holder 8 is formed from a resin member, an inner wall surface of the mounting hole 8a for receiving the pivot bearing member 6 may not always be finished with a high precision due to various factors such as shrinkage of the resin member when forming the bearing holder 8 with resin. Also, the mounting hole 8a may not be finished with a true circle in its cross section. As a result, the precision in positioning the pivot bearing member 6 in the radial direction may possibly lower, and the main body section 6a of the pivot bearing member 6 may not move smoothly even though the axial direction pressing force is applied by the flexible supporting member 9. As a consequence, the pivot bearing member 6 may not be highly accurately positioned in the axial direction. The lowered mounting precision of the pivot bearing member 6 not only results in generation of motor noise but also results in a larger fixing structure for retaining the bearing holder 8, which poses a difficulty in miniaturization of the motor.
To cope with the problems described above, some motors that have been developed have a structure in which the bearing holder 8 is formed from a pressed (forged) steel plate, as shown in FIG. 8, and the bearing holder 8 is affixed to portions of the yoke 5 of the stator section by welding. However, even when the steel bearing holder 8 is used. The mounting hole 8a may have problems in finishing precision in the surface condition, such as, for example, burrs that remain on its inner wall surface. Accordingly, like the bearing holder that is formed from a resin member described above, the steel bearing holder 8 has problems in that, for example, the main body section 6a of the pivot bearing member 6 does not move smoothly.